mibuokami on 11 August 2009
Prinny Bhaal in Disgaea, damn that thing takes a lot of grinding to kill.
Other honourable mention include C'thun from Wow.
He was the last end raid boss my Guild downed before the the expansion and aside from Nax is without a doubt the hardest raid WoW ever had for its time and also one of the most unique boss fight.
Final boss of most Touhou game are also ball-breaking in Lunatic mode.
